Method of storing natural gas for transport



Feb. 1, 1966 H. c. SECORD ETAL 3,232,725

METHOD OF STORING NATURAL GAS FOR TRANSPORT Mme/252E m V w E 3 N II- II.m d \4 3 H w \a n a l? O n F w m, n m H\\ \im \O o v u N 0 o h\ \m Tllll ll |L\ m wmnmmmmm ATTORNEYS United States Patent ()fifice 3,232,725Patented Feb. 1, 1966 3,232,725 METHOD OF STORING NAT GAS FOR TRANSPGRTHerbert Campbell Secord, Markyate Herts, England, and

Bernard J. Clarke, White Plains, N.Y., assignors to Vehoc Corporation,New York, N.Y., a corporation of New York Filed July 25, 1963, Ser. No.297,522 14 Claims. (Cl. 48-190) This invention relates to the storageand transportation of natural hydrocarbon gases and, more particularly,to a method whereby a natural gas is contained in a dense single-fluidstate suitable for transport, particularly by ship, at minimalcompression, refrigeration and containment costs per unit weight of gas.

Vast amounts of hydrocarbon gases are taken from oilfields in regions sofar removed or separated by water from sources of demand that much of ithas not heretofore been put to commercially profitable use. Certainheavier gases rich in propane and/or butane sometimes are recovered andtransported as liquids (L.P.G.), but the lighter natural gasesconsisting principally of methane are more often flashed off and burnedor vented at the wellhead. It is the broad purpose of this invention toprovide a new and improved method for storing and transporting theselighter hydrocarbon gases which are rich in methane and thus make theirenormous energy potential available in all parts of the world. Inparticular, the new method is intended to make use of ships by which gascan be transported in bulk.

Several methods have been proposed heretofore for the storage andshipment of light hydrocarbon gases rich in methane but none of them hasbeen entirely satisfactory. One common approach has been to liquefy thegas at the extremely low temperature (258 F.) at which pure methaneliquefies at atmospheric pressure. Though this concept of liquefyingmethane-rich natural gas produces about a 600-fold increase in density,the cost of refrigerating commercial quantities of the gas is virtuallyprohibitive in most practical applications.

Broadly stated, the invention provides a method of storing for transporta nautral gas containing at least 50 mol percent methane and at least 75mol percent methaneplus-ethane, the remainder being heavier hydrocarbonsand up to 20 mol percent inert constitutents, and having a grosscalorific value of from 800 B.t.u./s.c.f. to 1800 B.t.u./scf. (The grosscalorific value is intended in all references to B.t.u. ratingshereinafter set forth, and is that total or gross heating value obtainedby burning one cubic foot of the gas mixture at 14.7 p.s.i.a., with air,cooling the products of combustion to 60 F and condensing the moistureformed.) The steps of the method include compressing and refrigeratingthe gas to an operating state characterized by certain limits ofpressure and temperature. The maximum operating temperature is 20 P.less than ambient temperature but not above 32 F. The minimum operatingtemperature is about the critical temperature of methane. The maximumoperating pressure is 500 p.s.i. above the bubble point-dew pointpressure of the gas at operating temperatures less than the cricondenbarof the gas and is 500 p.s.i. above the cricondenbar pressure of the gasat operating temperatures in excess of the cricondenbar of the gas. Theminimum operating pressure is 50 p.s.i. above the bubble point-dew pointpressure of the gas at operating temperatures less than thecricondentherm of the gas and is 50 p.s.i. above the cricondenthermpressure of the gas at operating temperatures in excess of thecricondentherm of the gas. When the gas is thus compressed andrefrigerated, it is contained in the operating state to preventexpansion of the gas.

Then the contained gas is thermally insulated against heat leakage intothe gas so that it remains in the operating state throughout theduration of its containment. As a result, the gas is maintained in adense single-fluid phase without ullage suitable for transport atminimal compression, refrigeration and containment cost per unit weightof gas.

The term natural gas as used herein in regard to the method of theinvention is intended to include mixtures of hydrocarbon gasescontaining at least 50 mol percent methane and at least mol percentmethane-plus-ethane together with propane and butane and in mostinstances some heavier hydrocarbons and inert constituents. Such gaseswill have a gross calorific value of from 800 to 1800 B.t.u./sci. Thisdefinition of the term includes wellhead gases, gases separated fromcrude oil at a wellhead and tail gases from oil refineries and otherprocessing plants, but it excludes propane-butane mixturesconventionally handled in the liquid state as L.P.G. and artificiallyprepared solutions of pure methane dissolved in a heavier carrier suchas ethane.

Various natural gases are therefore suitable for the practice of thisinvention. For example, a Sahara gas having a critical temperature of 53F. and a critical pressure of 1144 p.s.i.a., with a calorific value of1200 B.t.u./s.c.f., has the following composition on a mol percentbasis:

Methane 83.92 Ethane 7.83 Propane 3.17 Isobutane 0.87 Normal butane 1.08Isopentane 0.55 Normal pentane 0.41 Hexane 0.52 Inerts 1.65

On the other hand, a typical Venezuelan gas having a criticaltemperature of +10 F. and a critical pressure of 1200 p.s.i.a., with acalorific value of 1534 B.t.u./s.c.f., is of the following mol percentcomposition:

Methane 61.37 Ethane 16.84 Propane 11.68 Isobutane 1.53 Normal butane4.29 Isopentane 0.84 Normal pentane 1.39 Hexane 0.61 Inerts 1.45

By containing natural gas in the operating state described previously,its density can be increased to a very great extent with surprisinglyminimal compression and refrigeration costs. As a result, the gas iscapable of being transported, particularly by ship, at far less expensethan by conventional atmospheric pressure liquefication processes.

The contemplated compression and refrigeration is particularly favorablefor shipment of rich hydrocarbon gases having relatively high criticaltemperatures (for example, 50 F. at 1200 B.t.u./s.c.f. to 0 F. at 1500B.t.u./s.c.f.), whereas with known liquefication processes it is noteconomical to carry much propane, butane or pentane along with methanedue to wasteful over-refrigeration of these components. Since allassociated gas and much dry field gas are rich in these heavierhydrocarbons, the present method is of great commercial importanceinasmuch as it economizes with higher operating temperatures and loweroperating pressures while transporting the valuable heavier componentsto the market for separation after shipment. Even leaner gases (from 800to 1200 B.t.u./s.c.f.), can be shipped more cheaply by the method of theinvention where the voyage is relatively short (less than 2000 miles)because the low fixed cost for refrigeration, which is independent ofdistance, much more than oifsets the higher containment costs on boardship due to higher pressures required and lower absolute densities.

The method of the invention may be better understood by referring to theaccompanying drawings, which is a pressure-temperature diagram of arepresentative natural gas showng the contemplated operating state.

Absolute values are not given in this diagram but the shape of thevarious curves is illustrative of a typical natural gas composition ofthe type described previously. The curve A-BCDE defines the envelopewherein the gas exists in a two-phase state, part liquid and part vapor.Point A indicates the liquefication temperature of the gas atatmospheric pressure and in absolute terms it might be approximately 258F. Point 13 is the true critical point of the gas at which the variouslines of uniform liquid and vapor concentrations within the two-phaseregion of the envelope converge. Point C is the cricondenbar of the gas,marking the point of highest pressure, regardless of temperature, atwhich the twophase condition can exist. Point D is the cricondentherm ofthe gas and it indicates the point of highest temperature, regardless ofpressure, at which the two-phase condition can exist.

From A to B the envelope curve is generally referred to as the bubblepoint line since it marks those definite equilibrium states where vaporwill begin to appear, for example during isothermal expansion of thegas. From the critical point B to the point E on the envelope, the curveis commonly referred to as the dew point line at which liquid begins tocondense, for example during isobaric cooling of the gas. Criticalpoints of representative natural gases contemplated for use in thismethod are at pressures of about 675 p.s.i.-a to 2000 p.s.i.-a. andtemperatures of about -l F. to +145 F.

Within the two-phase envelope ABCDE, it can properly be said that thegas exists as a liquid and a vapor" but outside the envelope it is bestthought of as a com pressible fluid regardless of pressure andtemperature since its physical state varies primarily with respect todensity. Thus, if the gas is compressed from the point X to Y and thencooled to Z, its density would gradually change without a distinctchange in phase. Only when changes in gas temperature and pressure arecarried out through the two-phase envelope, for example directly betweenX and Z, can the creation of a part liquid and part vapor condition bedistinctly noted. Therefore, the behavior of natural gas is referred toherein as that of a fluid whenever it is outside the two-phase region ofthe envelope, and by this is meant a compressible singlephase fluid.

In the broadest form of the new method, the gas is compressed andrefrigerated to an operating state circumscribed by the dotted linesconnecting points 1 to 11 on the diagram. Thus, the gas is to be broughtto a temperature below the dotted line connecting the points 6 and 7 inthe drawing, which should not be more than +32 F. and in no event inexcess of 20 F. below ambient temperature. By ambient temperature ismeant the environmental temperature at the point where the gas iscompressed and refrigerated into the operating state. Some refrigerationis therefore necessary in all forms of the contemplated method, if onlyby expansion cooling with work as the gas is loaded into containers. Thediagram also indicates the minimum operating temperature by the dottedline 11-1, which is the critical temperature of methane (116 F.). In nocase should the gas be cooled below about 116 F. in this new method ofstorage for shipment because refrigeration cost-s begin to rise steeply,the gain in density falls off, and the cost of metal containers with therequired high degree of notch toughness markedly increases attemperatures below that region.

The minimum operating pressure contemplated by the invention is shown inthe diagram by the dotted line connecting the points 1 to 6. It is neverless than 50 p.s.i. above the bubble point-dew point pressure of the gasat temperatures less than the cricondentherm D; and, at operatingtemperatures in excess of the cricondentherm D, the minimum operatingpressure is 50 p.s.i. above the cricondentherm pressure. The latter willbe an effective lower pressure limit only when the twophase envelopeABCDE does not project its criconden therm D beyond the maximumtemperature line 6 to 7, which may or may not be the case depending uponthe composition of the gas. It should be noted that the point 5 in thediagram is not actually on the envelope curve, but rather it marks apoint which is equal in temperature to the cricondentherm D but 50p.s.i. above it, and therefore point 5 is outside the envelope.

This parameter of minimum pressure insures that the gas will always bestored and transported in the singlephase compressible fluid state byavoiding entry into the two-phase envelope ABDE where an interface ofliquid and vapor will begin to appear. Ullage, a vapor pocket over theliquid, cannot therefore be present with its attendant disadvantage ofliquid slop, and indeed there is no need for it in the present method(as there is in LNG. and LPG. methods) because a rise of temperature isaccompanied by a tolerable rise in pressure in the singlephase highlycompressible fluid operating state. In addition to the problem of slop,operation with ullage in the two-phase region encounters greatly reduceddensities which prevent full use of costly containers.

The maximum operating pressure in the broad form of the method is shownin the diagram by the dotted line connecting the points 7 to 11. Atoperating temperatures less than the cricondenbar C, the upper pressurelimit is 500 p.s.i. above the bubble point-dew point line. The limitcontinues at operating temperatures above the cricondenbar C as 500p.s.i. above the cricondenbar of the gas. This upper limit on operatingpressure should not be exceeded because above it the cost ofcompressing, refrigerating and containing the gas becomes quiteuneconomical.

This broad form of the method may be utilized to transport a natural gasby ship. Once the gas is brought into the operating state it may beloaded into a multiplicity of metal pressure bottle containers closelystacked in the holds of large fast ships or barges. These containers areresistant internally to the chosen operating temperature and pressure.The holds should be internally insulated throughout to keep the gas andits containers essentially at the loading temperature throughout thedelivery voyage and also to keep the substantially empty containers nearthat temperature during the return voyage. At all times during loadingand unloading of the containers, the gas is maintained in thesingle-phase compressible fluid condition which characterizes theoperating state, in order to insure uniformity of composition andminimal undesirable temperature effects. Since the gas is best preparedat a relatively constant rate for such shipment and delivered withsimilar uniform flow rates to consumers, one ship is preferablyavailable for loading at all times while another is unloading and theremainder of the fleet plies between the two terminal ports. Thus, atleast four ships are required. This avoids the costs and double loadingand unloading operations from static storage tanks which otherwise wouldbe required at both ports. Such storage is of course not precluded wheresite conditions and special circumstances, such as delivery to severalterminals, favor it. In some instances, it may be profitable to addpropane to a leaner natural gas prior to shipment in order to obtain ahigher critical temperature for the mixture and thus to reduce theoperating pressure at the chosen operating temperature, the

propane being separated on delivery and returned on the ship to theloading port for re-use.

There is a further advantage in using this broad form of the method totransport natural gas by ship, as compared to conventionaltransportation by LPG. and L.N.G. techniques. Because of the relativelyhigh temperatures in the present method, heat leakage into the containedgas during the voyage is greatly reduced and far less in sulation, whichis costly both in itself and because of its bulk and weight, isrequired. The minor amount of heat leakage which does occur can be fullyoff-set by the use of some of the gas cargo as fuel for the ship or, ifoil fuel is cheaper, by a few degrees extra initial refrigeration. Inany case, there is never any need for positively induced refrigerationof the gas cargo on board the ship or of venting boil-off gas.

Within the broad range of maximum and minimum temperatures and pressuresdescribed previously, economic considerations in the practice of themethod may favor operation at the lowest temperature feasible withlow-alloy steel bottles. Therefore, it can be said that there is apreferred operating temperature range of from -50 F. to 80 F., thelatter being safely above (about 35 F.) the Nil Ductility Testtemperature for materials such as 1 to 2 percent nickel steel which hasbeen quenched and tempered to secure an ultimate tensile strengthapproaching 120,000 p.s.i. Such material has a thickness limitation ofabout 0.75 inch, and with a safety factor UT 8/ 3.2 this limits theoperating pressure accordingly to about 1340 p.s.i.a. in gas containershaving an outside diameter of 42 inches which is typical of presentdesign limitations for containers made with line pipe. For very leangases, it limits the operating pressure to about 1875 p.s.i. with gascontainers having a diameter of 30 inches.

The optimum operating conditions within these practical limits willdepend on the composition of the mixture to be shipped, the preferredworking pressure ranging from a minimum of about 600 p.s.i.a. for a 1750B.t.u./s.c.f. gas to about 1100 p.s.i.a. for a 1200 Bt.u./s.c.f. gas orto about 1800 p.s.i.a. for a 900 B.t.u./s.c.f. gas containing arelatively large amount of inerts. The minimum temperature of -80 F. inthis preferred range is so far below the critical temperature of veryrich gases that the singlephase fluid is relatively less compressibleand operation far above the bubble point pressure is less economical.With leaner gases having critical temperatures near or even below theminimum temperature of 80 F., operation considerably above the bubblepoint pressure may be more economical.

Having described the broad operating states contemplated in the presentmethod and also a relatively narrow region of operating temperaturetherewithin, some attention should be given to the basic subdivisions ofthe operating state distinguished by operation above and below thecritical temperature of the gas. In one form of the new method, the gasis refrigerated and compressed to an operating state wherein the maximumoperating temperature is the critical temperature of the gas and theminimum operating temperature is about 90 F. Within these temperaturelimits, the operating pressure [may vary depending upon the compositionof the gas mixture, but in all instances it will be at least 50 p.s.i.above the bubble point pressure of the gas at the operating temperatureand not in excess of 500 p.s.i. above the bubble point pressure of thegas at the operating temperature.

The accompanying pressure-temperature diagram indicates these limits.The maximum operating temperature is shown by the dotted line connectingthe points 3 and 9, the minimum operating temperature is shown by thedotted line connecting the points 2 and 10, the maximum operatingpressure is indicated by the dotted line connecting the points 9 and 10,and the minimum operating pressure is indicated by the dotted lineconnecting the points 2 and 3.

In restricting the operating temperature to below the criticaltemperature of the gas in this manner, it is preferable to achieve a 350to 400-fold increase in density of the gas relative to its density atatmospheric pressure and temperature. For example, a rich gas of 1535B.t.u./ s.c.f. and 0.9 specific gravity (relative to air) at F. has abubble point pressure of about 650 p.s.i.a. and it would preferably beloaded to a pressure of 750 p.s.i.a. to give a density of 27 lb./ft.which is 388 times its normal density and about the same density as thatof normally lean natural gas liquefied by refrigeration alone atsubstantially atmospheric pressure. On the other hand, a lean gas of1080 B.t.u./s.c.f. and 0.67 specific gravity would call for a workingpressure of about 1350 p.s.i.a. at 75 F., giving approximately 18lb./ft. and a 350- fold increase.

In another form of the new method, the minimum operating temperature isthe critical temperature of the gas and the maximum operatingtemperature is 20 P. less than ambient temperature but not above 32 F.These upper and lower limits on temperatures are shown respectively bythe dotted lines in the diagram connecting points 6 and 7 and points 3and 9. The operating pressure may again vary within these temperaturelimits depending upon the composition of the gas mixture, but in noevent will it exceed 500 p.s.i. above the dew point pressure of the gasat operating temperatures less than the cricondenbar temperature of thegas or 500 p.s.i. above the cricondenbar of the gas at operatingtemperature in excess of the cricondenbar temperature of the gas.Relating this to the diagram, the dotted line connecting the points 8and 9 indicates the maximum pressure at operating temperatures in thisform of the method below the temperature of the cricondenbar C and thedotted line connecting the points 7 and 8 indicates the maximum pressurefor operation at temperatures in excess of the cricondenbar C. Theminimum operating pressure in this embodiment is 50 p.s.i. above theupper dew point pressure of the gas at operating temperatures less thanthe cricondentherm temperature of the gas and is 50 p.s.i. above thecricondentherm pressure of the gas at operating temperatures in excessof the cricondentherm of the gas. Again relating this to the diagram,the dotted line connecting the points 3 to 5 indicates the minimumpressure at operating temperatures less than the cricondentherm D, andthe dotted line connecting the points 5 and 6 indicates the minimumpressure at operating temperatures in excess of the cricondentherm D. Asnoted previously, the minimum pressure indicated by the dotted lineconnecting the points 5 and 6 may be of no consequence in the event thecricondentherm D of the gas extends beyond the upper temperature limitindicated by the dotted line connecting the points 6 and 7.

For a better understanding of the invention, there follows a descriptionof the method as it applies to the transport by ship of the 1534B.t.u./s.c.f. Venezuelan gas mentioned previously. This gas is pipedfrom the wellhead along with its associated crude oil and is deliveredto separator facilities. Here the gas is flashed from the crude oil inthree stages of descending pressure and is dehydrated by a processemploying a molecular sieve. The dehydration treatment temperature isapproximately 120 F. After dehydration, the gas is compressed to 1765p.s.i.a. in centrifugal compressors. Subsequent to this compression thegas is cooled to R, which at that pressure maintains the gas completelyin the single-fluid phase.

Since the method of the invention particularly contemplates thetransportation of natural gas by ship, it may be necessary to carry thegas from the compression and dehydration facilities by means of apipeline to a shiploading platform. Depending upon the topographicalconditions where the method is practiced, this pipeline may traverseland or water or both. When the gas reaches the loading platform, it isreduced in pressure and tem- 7 perature to about 1500 p.s.i.a. and 80 F.and it is still in the single fluid phase. Thereafter, the gas isrefrigerated and expanded to its operating state of 950 p.s.i.a. and 50F. and a density of 24.7 pounds per cubic foot. A suitable refrigerantis propylene which is vaporized at atmospheric pressure and 54 F. Fromthat vaporized state the propylene is compressed to 258 p.s.i.a. and iscondensed to liquid at 105 F.v

Ships used to transport gas in accordance with this invention may carryin their holds a large number of pipe-like bottles capped at both ends.One such bottle may have an outside diameter of 42 inches, a wall of0.53 inch and a length of 50 feet. Upon completing the expansion andrefrigeration stage at the ship-loading platform, the gas is passed intothese bottles in the thermally insulated hold of a ship. The gas cargoin the loaded bottles is at a pressure of 950 p.s.i.a. and -50 F. andduring a voyage of several days this temperature will rise only about 2F. with no positive refrigeration. The slight thermal expansion whichdoes occur may be compensated by using the gas cargo as fuel for theships diesel engines. Thus, the gas Within the bottles may be at atemperature of about -48 F. and a pressure of about 875 p.s.i.a. whenthe ship arrives at its port of destination. One ship, for example, maytransport 37.5 million pounds of this 1534 B.t.u./s.c.f. gas in thismanner and when it is unloaded one million pounds may remain in thecontainers at 143 p.s.i.a. and 58 F. Allowing for some minor heatleakage during the return voyage and for fuel used in transit, the shipmay arrive back at the point of loading with 0.35 million pounds of thegas in its containers at 51 p.s.i.a. and -45 F.

Natural gas transported in accordance with this invention may beseparated at the point of destination essen tially to methane forcontinuous supply into a transmission system and heavier ends such asethane, L.P.G., and natural gasoline which may be piped separately toareas of use. The heavy ends may alternatively be converted mainly tomethane by exothermic reaction with steam over a nickel-containingcatalyst to augment further the pipe-line gas supply. In virtually allinstances, the net cost to the consumer of natural gas and itscomponents stored and transported over long distances pursuant to thismethod is substantially less than with any other process of watertransport of gas known heretofore.

While the invention is described herein as a method of storing naturalgas for transport, it applies also to th static storage of natural gas.

We claim:

1. A method of storing for transport a natural gas containing at least50 mol percent methane and at least 75 mol percent methane-plus-ethane,the remainder being heavier hydrocarbons and up to mol percent inertconstituents, and having a gross calorific value of from 800B.t.u./s.c.f. to 1800 B.t.u./s.c.f., which comprises:

(a) compressing and refrigerating said gas to an operating state wherein(i) the maximum operating temperature is 20 F less than ambienttemperature but not above 32 F.,

(ii) the minimum operating temperature is about the critical temperatureof methane,

(iii) the maximum operating pressure is :500 psi. above the bubblepoint-dew point pressure of the gas at operating temperatures less thanthe cricondenbar of the gas and is 500 psi. above the cricondenbar ofthe gas at operating temperatures in excess of the cricondenbar of thegas, and

(iv) the minimum operating pressure is 50 psi. above the bubblepoint-dew point pressure ot the gas at operating temperatures less thanthe cricondentherm of the gas and is 50 psi. above the cricondentherm ofthe gas at operating 3. temperatures in excess of the cricondentherm ofthe gas;

(b) containing said gas in the operating state to prevent expansion ofsaid gas; and

(c) thermally insulating the contained gas against substantial heatleakage into said gas so that it remains in said operating statethroughout the duration of its containment;

(d) whereby the gas is maintained in a dense singlefiuid phase withoutullage suitable for storage and transport at minimal compression,refrigeration and containment costs per unit weight of gas.

2. A method of storing a natural gas according to claim 1 wherein themaximum and minimum operating temperatures are -S0 F. and F.respectively.

3. A method of storing a natural gas according to claim 2 wherein thedensity of the gas in the operating state is 300 to 400 times greaterthan its density at atmospheric pressure and temperature.

4. A method of storing natural gas according to claim 1 wherein said gasis contained in said operating state in a multiplicity of metalcontainers resistant to the chosen operating temperature and pressure,and said containers are surrounded by thermal insulation and transportedby ship.

5. A method of storing natural gas according to claim 4 wherein said gasis loaded in and unloaded from at least four of said ships sequentiallyat a substantially uniform rate without static storage at the points ofloading and unloading.

6. A method of storing for transport a natural gas containing at least50 mol percent methane and at least 75 mol percent methane-plus-ethane,the remainder being heavier hydrocarbons and up to 20 mol precent inertconstituents, and having a gross calorific value of from 800B.t.u./s.c.f. to 1800 B.t.u./s.c.f., which comprises:

(a) compressing and refrigerating said gas to an operating state wherein(i) the maximum operating temperature is the critical temperature of thegas,

(ii) the minimum operating temperature is about (iii) the maximumoperating pressure is 500 p.s.i. above the bubble point pressure of thegas at the operating temperature, and

(iv) the minimum operating pressure is 50 p.s.i. above the bubble pointpressure of the gas at the operating temperature;

(b) containing said gas in the operating state to prevent expansion ofsaid gas; and

(c) thermally insulating the contained gas against substantial heatleakage into said gas so that it remains in said operating statethroughout the duration of its containment;

(d) whereby the gas is maintained in a dense singlefluid phase withoutullage suitable for storage and transport at minimal compression,refrigeration and containment costs per unit weight of gas.

7. A method of storing a natural gas according to claim 6 wherein thedensity of the gas in the operating state is 350 to 400 times greaterthan its density at atmospheric pressure and temperature.

8. A method of storing natural gas according to claim 6 wherein said gasis contained in said operating state in a multiplicity of metalcontainers resistant to the chosen operating temperature and pressure,and said containers are surrounded by thermal insulation and transportedby ship.

9. A method of storing natural gas according to claim 8 wherein said gasis loaded in and unloaded from at least four of said ships sequentiallyat a substantially uniform rate without static storage at the points ofloading and unloading.

A method Of StOring for transport a natural gas containing at least 50mol percent methane and at least 75 mol percent methane-plus-ethane, theremainder being heavier hydrocarbons and up to 20 mol percent inertconstitutents, and having a gross calorific value of from 800B.t.u./s.c.f. to 1800 B.t.u./s.c.f., which comprises:

(a) compressing and refrigerating said gas to an operating state wherein(i) the maximum operating temperature is 20 P. less than ambienttemperature but not above 32 F (ii) the minimum operating temperature isthe critical temperature of the gas,

(iii) the maximum operating pressure is 500 p.s.i.

above the dew point pressure of the gas at operating temperatures lessthan the cricondenbar of the gas and is 500 p.s.i. above thecricondenbar of the gas at operating temperatures in excess of thecricondenbar of the gas, and

(iv) the minimum operating pressure is 50 p.s.i.

above the dew point pressure of the gas at operating temperatures lessthan the cricondentherm of the gas and is 50 p.s.i. above thecricondentherm of the gas at operating temperatures in excess of thecricondentherm of the gas;

(b) containing said gas in the operating state to prevent expansion ofsaid gas; and

(c) thermally insulating the contained gas against substantial heatleakage into said gas so that it remains in said operating statethroughout the duration of its containment;

(d) whereby the gas is maintained in a dense singlefiuid phase withoutullage suitable for storage and transport at minimal compression,refrigeration and containment costs per unit weight of gas.

11. A method of storing natural gas according to claim 1 wherein saidgas includes among its heavier hydrocarbons an additive of propane toincrease the critical temperature of the gas and to lower the minimumoperating pressure or the method.

12. A method of storing a natural gas according to claim 10 wherein themaximum operating temperature is F. and the density of the gas in theoperating state excluding any inerts is 300 to 3 50 times greater thanits density at atmospheric pressure and temperature.

13. A method of storing natural gas according to claim 10 wherein saidgas is contained in said operating state in a multiplicity of metalcontainers resistant to the chosen operating temperature and pressure,and said containers are surrounded by thermal insulation and transportedby ship.

14. A method of storing natural gas according to claim 13 wherein saidgas is loaded in and unloaded from at least four of said shipssequentially at a substantially uniform rate without static storage atthe points of loading and unloading.

References Cited by the Examiner UNITED STATES PATENTS 2,217,678 10/1940Goosmann. 2,231,500 2/1941 Harlow.

OTHER REFERENCES Katz et 211.: Industrial and Engineering Chemistry,vol. 32, No. 6, pages 817-827 (June 1940).

MORRIS O. WOLK, Primary Examiner.

Dedication 3,232,725.Herbert Campbell Secm'd, Markyate Herts, Englandand Bernard J. Clarke, W'hire Plains, NY. METHOD OF STORING NATURAL GASFOR TRANSPORT. Patent dated Feb. 1, 1966. Dedication filed Sept. 16,1971, by the assignee, Vehoo Corporation. Hereby dedicates to the Publicthe entire remaining term of said patent.

[Ofiicial Gazette December 28, 1.971.]

1. A METHOD OF STORING FOR TRANSPORT A NATURAL GAS CONTAINING LAT LEAST50 MOL PERCENT METHANE AND AT LEAST 75 MOL PERCENT METHANE-PLUS-ETHANE,THE REMAINDER BEING HEAVIER HYDROCARBONS AND UP TO 20 MOL PERCENNT INERTCONSTITUENTS AND HAVING A GROSS CALORIFIC VALUE OF FROM 800B.T.U./S.C.F. TO 2800 B.T.U./S.C.F., WHICH COMPRISES: (A) COMPRESSINGAND REFRIGERATING SAID GAS TO AN OPERATING STATE WHEREIN (I) THE MAXIMUMOPERATING TEMPERATURE IS 20*F. LESS THAN AMBIENT TEMPERATURE BUT NOTABOVE 32*F., (II) THE MINIMUM OPERATING TEMPERATURE IS ABOUT THECRITICAL TEMPERATURE OF METHANE, (III) THE MAXIMUM OPERATING PRESSURE IS500 P.S.I. ABOVE THE BUBBLE POINT-DEW POINT PRESSURE OF THE GAS ATOPERATING TEMPERATURES LESS THAN THE CRICONDENBAR OF THE GAS AND IS 500P.S.I. ABOVE THE CRICONDENBAR OF THE GAS AT OPERATING TEMPERATURES INEXCESS OF THE CRICONDENBAR OF THE GAS, AND (IV) THE MINIMUM OPERATINGPRESSURE IS 50 P.S.I. ABOVE THE BUBBLE POINT-DEW POINT PRESSURE OF THEGAS AT OPERATING TEMPERATURES LESS THAN THE CRICONDENTHERM OF THE GASAND IS 50 P.S.I. ABOVE THE CRICONDENTHERM OF THE GAS AT OPERATINGTEMPERATURES IN EXCESS OF THE CRICONDENTHERM OF THE GAS; (B) CONTAININGSAID GAS IN THE OPERATING STATE TO PREVENT EXPANSION OF SAID GAS; AND(C) THERMALLY INSULATING THE CONTAINED GAS AGAINST SUBSTANTIAL HEATLEAKAGE INTO SAID GAS SO THAT IT REMAINS IN SAID OPERATING STATETHROUGHOUT THE DURATION OF ITS CONTAINMENT; (D) WHEREBY THE GAS ISMAINTAINED IN A DENSE SINGLEFLUID PHASE WITHOUT ULLAGE SUITABLE FORSTORAGE AND TRANSPORT AT MINIMAL COMPRESSION, REFRIGERATION ANDCONTAINMENT COSTS PER UNIT WEIGHT OF GAS.